A device to operate a turn-off thryistor is described in the periodical "Elektrotechnik", issue 4, 1983, at pages 14 through 17, and FIG. 12, where a converter is shown connected in a bridge circuit. A control source is connected to the turn-off thyristor which is connected on its output side to the gate connection and to the cathode connection of the turn-off thyristor. The control source controls the thyristor as a function of a switch-off command or a switch-on command when in a power-conducting or power-blocking state respectively. The block diagram of the control source is also shown in the periodical "Elektrotechnik", issue 24, 1982, pages 16 through 21 in FIG. 7. To assure reliable operation of the turn-off thyristor, current and voltage at the turn-off thyristor have to remain within preset limits. For that purpose, in the known apparatus the design incorporates a capacitor parallel to the turn-off thyristor, a resistance and two diodes; moreover, wired in series with the turn-off thyristor there is an inductor and a current sensor device which detects the current through the turn-off thyristor which in case of excess current issues a switch-off command to the control source.
In the existing device it is necessary for the above-described circuitry to be designed to limit the current and voltage at the turn-off thyristor to the worst case operating condition of the turn-off thyristor in the converter in order to be able to continually assure safe operation of the turn-off thyristor; however, these circuitry parameters contradict the requirement for rapid switch-on and switch-off of the current by the turn-off thyristor. Thus, for example, the inductor connected in series with the turn-off thyristor is rated so that it limits the current rise through the turn-off thyristor in case of excess current by making possible a switch-off of the turn-off thyristor by means of the current sensor device before the current attains a maximum permissible conducting state current value that can be switched off by the turn-off thyristor. This arrangement with the inductor involves a relatively major additional circuitry expense and, moreover, results in additional power losses and voltage drops.
In order to avoid a short circuit in the converter after issuing a turn-off command to the control source controlling the turn-off thyristor, a triggering of the additional turn-off thyristor in the same bridge circuit into a conductive state is only possible once the turn-off thyristor has totally shut off the current. It is thus required for reliable operation of the turn-off thyristor as a function of the circuitry at the turn-off thyristor and its delayed reaction, to specify an adequately calculated interlock time for the worst case operating situation upon the issuance of the turn-off command, during which time any triggering of the additional turn-off thyristor into a conductive state is inhibited. Observing this interlock time, however, produces unfavorable operating characteristics of the converter.
The object of this invention is thus to develop a procedure and device that provides both particularly reliable and optimal operation of the turn-off thyristor in terms both of its utilization and of its reliability protection.